This invention relates to a unique signal processing scheme in the digital traffic radar field and represents an improvement in the art over that generally shown in the patents to Fathauer, U.S. Pat. No. 3,438,031; Berry, U.S. Pat. No. 3,689,921 and Aker, et al., U.S. Pat. No. 3,936,824. This signal processing scheme includes a means for gathering Doppler signal as a digital basis after the squaring and arraying of same in the memory of a microprocessor unit. The term arraying will be described in detail in a later part of this disclosure; however, for purposes of this brief discussion, the term "arraying" may be thought of as collecting a series of samples of the Doppler signal and causing the samples to be "arrayed" within a series of memory locations. Following this arraying, the associated microprocessor will examine the array and by executing a predetermined algorithm, a decision will be made as to whether a valid signal, suitable for computing a display number as an indication of speed, exists within the array.
The invention also includes a unique tracking filter system which is controllable over the expected Doppler frequency range. It has "duty factor modulation" as a means for controlling the effective values of the RC time constants within the filter elements. This filter has a band-pass system which is used for tracking the patrol car (host vehicle) speed (Doppler signal). It also has a high pass filter with a Bessel-type high pass function which contains a notch (zero of response) at the edge of its stopband. The operation of the system is generally such to place the notch on the patrol car speed frequency so that the patrol car speed signal is removed from the composite signal and passed onto the signal processing elements. These two filters are also under microprocessor control and are selected and controlled by it depending upon which operation is being performed.
Known moving radar prior art has various means for determining signal validity. The first known commercially successful digital police radar is believed to be that shown in the Berry Pat. No. 3,689,921 which incorporated frequency counting means for display and unique signal validation means for determining when the measurement of the Doppler signal (speed of the target vehicle) was suitable for display. Radar units constructed in accordance with the teachings of the Fathauer Pat. No. 3,438,031 used a frequency counting means but incorporated a different signal validation system. In any event, the signal validation system must be one that will substantially eliminate false readings. If not, the radar's operations will not be judicially accepted, which is highly detrimental to its commercial success.
One of the improvements of the Berry radar over the Fathauer device was in the mulitple comparison scheme which allowed the radar to operate in a law-enforcement environment with an acceptably low rate of false or ghost readings. The Aker, et al U.S. Pat. No. 3,936,824 for moving radar expanded the art shown by the Berry radar from a stationary application to one in which the radar was operable from a moving police vehicle. The range of the early moving radar device was initially acceptable (although lower than an equivalent stationary type radar), primarily because radar detectors had not yet become a commonly used item in the motorist's possession.
In a moving radar device, the combined speed of closure makes the available time the target vehicle is within a displayable range relatively short compared with stationary radar devices. Therefore, it is highly desirable to increase the radar's range as much as possible, given the various design parameters of RF output power, radar detector sensitivity, signal to noise ratio, operation in a multiple target environment and operation with ghost free readings.
Some of the most recent radar devices, such as that disclosed in the co-pending Berry patent application, filed Oct. 18, 1977, and bearing Ser. No. 843,259, now abandoned, operate using a phase lock loop for determining the noise quality of the signal.
The subject disclosure has a unique signal processing scheme which enables it to operate with only a short burst of good signal on a distant vehicle or with a signal which is superimposed upon noise or multiple vehicle Doppler signals.
One of the characteristics of the Doppler signal, when a target vehicle begins to approach a range in which the signal can be detected, is that the signal begins to build up and then drop off and then build up to a higher amplitude and then drop off. All known previous art devices require that the signal be valid (whether using a phase lock loop, a counting process or a threshold detector) for a period of time sufficient for the Doppler counting system to complete a full cycle. For example, in X-band this is normally a 31 millisecond time interval to complete one count cycle corresponding to the time base interval, e.g., this allows a count of 60 to be entered into the digital counter if the target vehicle is moving with respect to the radar at a rate of 60 miles per hour. Accordingly, an approaching target vehicle has to close to a range where its detected Doppler signal consistently remains above this good threshold level for several time base intervals. Fading effects are generally caused by the noise and Fresnel effect (multipath cancellations of the microwave signal, primarily off the highway surface). The present invention allows the signal to be examined in short bursts and the array processing determines the validity of the short bursts that were sampled. For example, in searching for a valid signal, the present invention can make a determination on as few as 32 waveform cycles of the oncoming Doppler signal. Depending on which band (X or K) the radar is operating within, this may be a very short time interval. For example, at the X-band frequency of 10.525 gHz, a 100 mile an hour signal is typically 3140 cycles. This means that a 32 cycle sample requires only 1 1/100th of a second or roughly 10 milliseconds. At K-band, a 32 cycle sample would require only about 40% of this time or approximately 4 milliseconds of sampling. In the present invention, the time required to take a 32 cycle sample decreases as the frequency of the target Doppler increases. This is particularly beneficial in view of the fact that the flutter rate of the Fresnel cancellations also increase with closing speed.
Another aspect of this invention is the ability to turn the microwave radiation on and off by microprocessor control. When the signal is not being collected, but is rather being processed, the system has the capability of turning the RF off, thereby making it not detectable by radar detectors.
The Aker, et al. U.S. Pat. No. RE 29,401 teaches that a moving radar could be operated in an environment with the proper filtering means for separating the low Doppler or patrol car speed Doppler signal from that of the composite speed Doppler signal and then an arithmetic subtraction can be accomplished between the two measurements. The filtering scheme disclosed in the Aker, et al U.S. Pat. (RE 29,401) used several fixed filters which were tunable or selected over several fixed frequencies. However, these filters were discretely selectable in frequency and not continuously selectable. To maximize the effectiveness of the array processing scheme, it is necessary to accomplish good separation of Doppler signal (from low to high) so that one signal does not restrict the performance or the detection of the other. Since both Doppler signals co-exist in any moving radar environment, the unique filtering scheme disclosed in the present invention is continuously tunable as opposed to discretely tunable, thereby allowing the microprocessor to control or place the filtering frequencies anywhere over the expected Doppler range. As will be seen, the notch or null point of the high pass can be placed by the microprocessor exactly on the patrol car speed Doppler signal so as to null it out completely.
The implementation of tunable elements in the above-mentioned filters is actually accomplished by duty factor modulating of resistor values. Each resistor, which forms one of the characteristic poles of zero of the filter, is connected in series with a transmission gate. These transmission gates are CMOS, having a very low on resistance and offering suitable isolation while in the off state. The transmission gates to each filter are connected to a one shot multivibrator, which is triggered each time a frequency crossing of suitable polarity occurs on the control signal. Since the one shot triggers which constant pulse width duration, at a low control frequency, the output or on condition has a comparatively low ratio or duty factor with respect to the off condition. As the control frequency increases, the duty factor of the one shot output increases up to the maximum value of 1.0.
In the present invention, the master clock frequency (or repetition rate of the one shot) is 32 times the rate of the Doppler signal which is expected to be processed. This rate should be as high a multiple of signal frequency as possible, given the restrictions due to the delay constants of the transmission gates.
The output of a band pass filter and the output of a high pass filter each go to low pass networks which remove the capacitive spikes coupled through the transmission gates as they are turned on and off. This signal is then passed to a squaring amplifier which converts the signal into a high amplitude digital waveform. The output of each filter, now in digital form, can be selected for validation by a digital device based on nonamplitude characteristics. In other words, the amplitude is two-valued and the signal will be examined by the array processing for periodicity (having a periodic nature) and for noise contributions.
As will be seen, the master oscillator feeds the two one shots that control the frequencies of the band pass and the high pass filters. The high pass filter one shot can be driven from the master oscillator or from a slave VCO. The selection of master or slave is accomplished by the microprocessor depending on the desired frequency of operation of the filter network.
The subject invention is designed to operate in a way that allows the RF energy to be turned on or off under the control of the microporcessor. This enables the unit to operate in a radar detector avoidance mode.
In the moving mode of operation, the operator is given a remote control which he can place in his lap or on the seat beside him and which allows him to press the button and put the unit into the hold mode. In the hold mode of operation, the RF enerby turns off and the unit displays the last patrol car speed in the patrol window with flashing means of presentation and says "HLD" in the target vehicle window of the dispaly. Periodically, at random or pseudorandom intervals, the RF is turned on for a brief instant which allows the unit to verify that it is travelling at the same rate of speed that it was a few seconds ago. This allows it to maintain acquisition of the ground speed of the patrol vehicle so that when a target vehicle approaches to a point where the operator is sure that it is within range, the operator can then touch a button on the seat beside him and the radar will turn on the RF energy and within a few hundred milliseconds lock onto the target vehicle. The hold mode of operation allows the unit to maintain ground speed and not have to search for ground speed as an ordinary moving radar would have to be if it was merely turned off and then turned back on. In such a case, it is first necessary to acquire ground speed and then, after the necessary delay to accomplish this, the target speed could then be measured. This from the time RF energy first becomes apparent allows a person equipped with a radar detector time to slow his vehicle down before the radar can acquire its target.
Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description of the drawings.